Documents of value such as passports, identification cards, driver's licenses, entry passes, ownership certificates, financial instruments, and the like, are often assigned to a particular person by personalization data. Personalization data, often present as printed images, can include photographs, signatures, fingerprints, personal alphanumeric information, and barcodes, and allows human or electronic verification that the person presenting the document for inspection is the person to whom the document is assigned. There is widespread concern that forgery techniques can be used to alter the personalization data on such a document, thus allowing non-authorized people to pass the inspection step and use the document in a fraudulent manner.
A number of security features have been developed to help authenticate a document of value or object of value, thus assisting in preventing counterfeiters from altering, duplicating or simulating a document of value or object of value. Some of these security features may include overt security features or covert security features. Overt security features are features that are easily viewable to the unaided eye. Such features may include holograms and other diffractive optically variable images, embossed images, and color-shifting films. In contrast, covert security features include images only visible under certain conditions, such as inspection under light of a certain wavelength, polarized light, or retroreflected light. One example of a laminate that may include both overt and covert security features is 3M Confirm™ Security Laminate, which is commercially available from 3M Company of St. Paul, Minn. This security laminate may be used with documents of value, such as identification cards, badges, and driver licenses, or objects of value, and assists in providing identification, authentication, and to help protect against counterfeiting, alteration, duplication, and simulation. Another example of a laminate that includes both overt and covert security features is illustrated in U.S. Pat. Publication No. 2003/0170425 A1 “Security Laminate” (Mann et al.). Examples of some other devices are taught in U.S. Pat. Nos. 3,801,183 and 4,688,894.
WO 09/105,142 describes an example of a security laminate which includes a first substrate having a first major surface and a second major surface. A plurality of retroreflective elements are affixed along the first major surface of the substrate. The retroreflective elements include a solid spherical core having an outer core surface, and the outer core surface provides a first interface. The retroreflective elements also include a first complete concentric optical interference layer having an inner surface overlying the core surface and an outer surface, the outer surface of the first complete concentric optical interference layer providing a second interface. The retroreflective elements further include a second complete concentric optical interference layer having an inner surface overlying the outer surface of the first complete concentric optical interference layer and an outer surface, the outer surface of the second complete concentric optical interference layer providing a third interface. The security laminate is retroreflective.
WO 01/29587 describes an exposed-lens retroreflective article exhibiting retroreflection under dry and wet conditions. The article comprises a first set of optical elements having a first reflective layer disposed on the embedded portion of the elements. The first set contributes mainly to dry retroreflection. The article further comprises a second set of optical elements having a second reflective layer behind a spacing layer. The second set contributes mainly to wet retroreflection. In one embodiment, the first and second sets of optical elements have substantially the same average diameters, refractive index, and density.
U.S. Pat. No. 5,777,791 describes a pavement marking article exhibiting dry and wet reflective characteristics. The article comprises a first set of optical elements having a refractive index between 1.9 and 2.0 protruding from a binder layer having particles of specular reflective pigment. These elements mainly contribute to dry reflection. The article further comprises a second set of optical elements having a refractive index between 2.2 and 2.3 which mainly contribute to wet reflection.
Although the commercial success of available security features has been impressive, as the capabilities of counterfeiters continue to evolve, it is desirable to further improve the ability to indicate that a security feature has been tampered with or somehow compromised to help protect against counterfeiting, alteration, duplication, and simulation.